The Rotating Disk Electrode Emission Spectroscopy Technique

Malte Lukas
Tags: wear debris analysis, oil analysis

In effort has been made by a task group within the American Society for Testing and Materials (ASTM) Committee D-2 on Petroleum Products and Lubricants to develop a test method to determine wear metals and contaminants in used lubricating fluids by rotating disc electrode (RDE) atomic emission spectroscopy. In principle, this sounds like a straightforward task. However, the fact that the RDE technique already has such wide spread use in commercial, military and industrial laboratories makes it more difficult to get agreement on what a “standard method” should be, versus that which has been practiced for several decades.

The RDE technique has experienced a recent rebirth due to instrumentation improvements and enhanced capabilities. The major attributes of the RDE technique, i.e., no sample preparation, fast and simultaneous analysis, simplicity, mobility of the spectrometer and readily available consumables continue to expand its use. Since the RDE technique for oil analysis has been in use for over 40 years, an obvious question might be “why do you need a standard method?” The answer is simple, there is no recognized standard procedure for RDE spectrometers that is universally applied or recognized. The need for a standard test method has been consumer driven and requested by numerous oil analysis laboratories. In these days of quality conscious consumers and ISO certification, standard test methods are a means to assure a consumer that testing is performed by an accepted and traceable procedure. Although it is an established technique, there is no standard test method that all laboratories can adhere to.

ASTM is the world’s foremost developer and provider of voluntary consensus standards and related technical information that contribute to the reliability of systems and services1. Petroleum products and lubricant related standards are the responsibility of ASTM Committee D-2. Within D-2, Subcommittee 3 on Elemental Analysis is responsible for the development of standards for the chemical determination of elements in petroleum products. A task group was thus formed within the Elemental Analysis subcommittee to write a new RDE test method.

The procedure for developing and adopting new test methods is very thorough and, on average, takes three years. A task group made up of key stakeholders in the new method is formed to write a draft in accordance with the form and style specified by ASTM. When the task group is satisfied with the draft, an inter-laboratory test program is initiated with actual oil samples to test and evaluate the method to show its precision and suitability as a laboratory test. After a satisfactory inter laboratory test, the task group can recommend the new method for a subcommittee ballot. If the subcommittee ballot is satisfactory and without any negative votes, the new method is submitted to ASTM Headquarters for D-2 ballot. Once accepted as a new test method, it is reviewed every four years and revised as appropriate.

The proposed test method for used oil analysis by the RDE technique is well on its way to approval by ASTM. Over the past two years, several drafts have been written and the required inter-laboratory study has been successfully completed. Worth mentioning is the fact that 14 very different laboratories from all over the world participated in the inter-laboratory study. The range included commercial testing laboratories, power generation companies, oil companies, automotive manufacturers and a transportation company. Although they have similar objectives, their methodology, instrument model and training were very different. The inter-laboratory study did show that a new method could be created and applied universally in an effort to standardize an existing technique to create conformance of analytical data. Most recently the proposed test method passed the subcommittee ballot without any negative votes. The next and final step is the full ASTM D-2 ballot scheduled for later this year.

The foremost challenge in drafting a test method for used oil analysis by the RDE technique was to obtain agreement from the variety of long-time users. The wide spread use and acceptance of the RDE technique has lead to a de facto acceptance of the method, yet in all these years, no attempts were made to develop an accepted test method in order to standardize it. It was thus not unexpected to discover that the RDE technique was not always applied in the same manner. The differences were, however, not all that drastic and users were willing to compromise in the interest of standardization and the creation of an approved ASTM test method. A final and approved test method for used oil analysis by the RDE technique is anticipated for 2001.

1 ASTM Committee D-2 on Petroleum Products and Lubricants, “Facts for Members”, ASTM, December, 1997.